Motor control



G. F. DRAKE MOTOR CONTROL July 12, 1938.

Filed March 23, 1934 INVENTOR George fbrrest .DraKe MAJ 1 w mv ATTORN EYS Patented .Iuly 12, 1938 UNITED STATES MOTOR CONTROL George Forrest Drake, Rockford, Ill., assignor to Howard D. Colman, Rockford, Ill.

' Application March 23,1.934, Serial No. 716,927

10 Claim.

This invention relates to the control of reversible electric motors and the primary object is to provide a novel system by which the direction of rotation of an electric motor may be reversed by opening and closing a single switch directly controlling the motor control circuits. 0

The invention also resides in the novel character of the means employed in carrying out the foregoing object.

Another object is to provide a novel motor control of the above character which is especially adapted to be governed by a sensitiveautomatic instrumentality such as a thermostat.

Other objects and advantages of the invention will become apparent from the following detailed description taken in connection with the accompanying drawing, in which Figure 1 is an enlarged elevational view partially in section of an electric motor and a control therefor embodying the features of'the present invention.

Figs. 2, 3, and 4 are wiring diagrams illustrating modifications of and alternative ways of utilizing the invention. I

As an example of a motor adapted tobe controlled in accordance with the present invention, I have shown in the drawing a reversible motor of the so-called shaded pole induction type which includes a field member 6 of the core type having a main winding I to be energized by alternating current and providing two polar projections defining a cylindrical recess in which is disposed a rotor 8 of the squirrel-cage type. The induction bars ll are left exposed at the rotor periphery thereby contributing to the low reactance of the rotor and poor single phase characteristics of the motor. I

Shifting of the magnetic field around the rotor recess, in one direction or the other to initiate and maintain rotation of the rotor in corresponding directions is produced by the'well known action ,offshading or short-circuited coils l0 and II enclosing side portions I2 of the pole pieces preferably'leaving intermediate portions l3 of the pole pieces unenclosed. The coils on corresponding side portions of the respective poles are connected in series relation and each coil is composed of a relatively large number, approximately 5001 turns of No. 34 standard gauge wire in the present instance, so that the eirternal resistance of the means bywhich the coils are controlled is a small part of that of the shading coils and does not therefore materially affect the motor operation. For the purpose of prope'rlydistributing the magnetic field for efiective utilization in the rotor,

the adjacent side tips of the different poles are joined by magnetic extensions ll of narrow radial width having internal surfaces closely following the rotor surface.

With the shading coils thus arranged, the magnetic field threading the coils when they are short-circuited with the winding 1 excited will induce currents in these coils which currents react with the main field flux in a well known manner to produce a lagging of the flux entering the rotor from the portions of the poles enclosed by the short-circuited coils relative to the unshaded fiux entering the rotor from the remaining portions 01 the poles. Thus, when the coils II) are short-circuited and the coils H are inefi'ectual, the magnetic field shifts around the rotor in a clockwise direction causing the rotor'to turn in this direction. Similarly, counter-clockwise rotation occurs when the coils II are effectively short-circuited.

It wi1l.be observed that the motor above described may be operated in either direction by selectively controlling the two windings l0 and H. The present invention contemplates an arrangement of the control circuits of the two starting windings in a manner such that one or the other may be rendered effective selectively by means of a single switch. For this purpose, the circuit for one of the windings is constructed to possess electrical characteristics different from the other circuit whereby the motor will run in one direction when one winding is eflective and in the opposite direction under the predominating effect of the other winding when both 01' the windings are effective.

For the sake of simplicity, it is preferred to vary the relative resistances of the circuits for the two windings, for example by interposing a separate resistance element 15 in series with the coils lo, the value of the resistance being such that the motor will run in a clockwise direction when the coils Ill alone are short-circuited through the resistance and in the reverse direction under the predominating shading effect of the coils II when both sets of coils are shortcircuited. As shown in Fig. l, the resistance element I5 is connected directly to the terminals of the shading coils In by conductors l6 and H, the latter being common to one terminal of the coils II. The conductor I1 extends to one terminal of a control switch l8, .the other terminal of which is connected by a conductor I9 to the insulated terminal of the coils I I. This switch may be actuated by a sensitive automatic instrumentality such as a thermostatic element 20. The shading coils ill may thus be short-circuited permanently through the resistance It and the circuit through the coils ii is controlled by the switch l8. When the switch is open, the coils ill will be eflective to produce clockwise rotation of the rotor while, when the switch is closed, the coils II will, owing to the lower resistance oi. their circuitsproduce a predominating shading eflect causing counterclockwise rotation.

Preierably, the value of the resistance elementll is such that the motor will develop substantially the same torque in both directions of rotation. For any given motor, this value is determined by the characteristics oi the motor and the external resistance of the control circuits.

I! desired. the relative eflectiveness of the control windings may be varied in other ways. For example, the resistance of one oi! the circuits may be increased by decreasing the size of wire from which the coils therein are wound. Or as illustrated in Fig. 2, the coils it may have a smaller number 01 turns than the coils ii although the resistances oi the two circuits may be the same.

The control arrangement above described is especially advantageous in increasing the sensitivity of control in systems where the controlling instrumentality is a thermostat, hygrostat or the like. With the present control, it is only necessary for the sensitive element to move through a short distance or accumulate a small amount of energy in order to reverse the motor owing to the use of the single control switch. Moreover,

the burden on the control switch is reduced and the service liie thereoi prolonged by utilizing the control in conjunction with a shaded pole motor of the character above described wherein the power consumed in the circuits by which the motor is controlled is only a small part, about one twentieth in the present instance, of the total power input oi! the, motor. For many of these applications, it is desirable to interpose limit switches 2| in the shading coil circuits, these being operated mechanically and respectively arranged to be opened at opposite limits in the movement of the device actuated by the motor.

Fig. 3 illustrates an application of the present system of control wherein the circuit through the shading coils II is controlled by the thermostatic switch l8 and that through the coils I is governed by an independently operable thermostatic switch 22. With this arrangement, the switch 22 controls the operation of the motor in one direction only while the switch [8 governs both directions of rotation. The rotation in one direction under the control of the switch I8 is, however, dependent upon the switch 22 being closed so that the latter switch, when open, prevents operation of the motor in one direction under the control of the switch l8.

For some applications, it is desirable to provide for transferring the resistance element from one shading coil circuit to the other whereby to vary the control by auxiliary control switches. An arrangement for this purpose is shown in Fig. 4 wherein two switches 23 and 24 are arranged to be controlled by the common arm of a thermostatic element 25 and switches 26 and 21 are similarly arranged for actuation by an independent thermostatic element 28. A conductor 29 leads to the common contacts oi. both sets of switches, directly in one case and indirectly through the medium of the resistance element I in the case of the switches 26 and 21. One insulated contact of the switch 21 is connected by a conductor II to the shading coils II and by a conductor 3| to the insulated contact oi the switch 2!. The switch 22 is connected to the coils Ii by a conductor 32 which is joined to a conductor 28 leading to the switch 28.

With the arrangement last described, it will be observed that when the switch 21 is closed and the'resistance ll included in the circuit of the coils II, the motor is under the control of the switch 22 and will run in one direction when the switch is closed to short-circuit the coils i l and in the opposite direction when the switch is open, the coils ll then being eiiective. Rotation in the latter direction will be interrupted whenever the switch 21 is opened. Assume now that the thermostat 28 closes the switch 26 thereby interposing the resistance It in the circuit of the coils ll. Then the motor will be under the control of the switch 24, the shading coils Ii and II being effective when the switch is opened and closed respectively.

I claim as my invention:

1. The combination of a shaded pole induction motor having a main winding adapted to be en ergized by alternating current and a pair of shading coils respectively adapted when short-circuited with said main winding energized to cause shifting of the magnetic field around the rotor in opposite directions, a circuit for one of said coils having a control switch therein, a circuit for the other coil adapted to be closed indeperidently 01' said switch, the resistance 01 said last mentioned circuit being greater than that of the first mentioned circuit, the ratio of the circuit resistances being such that the shadingeffect of said last mentioned coil will cause said rotor to turn in one direction when said switch is open and the predominating shading eiIect of said first mentioned coil will initiate reverse rotation when both of said circuits are closed whereby to render the motor reversible by opening and closure of said switch.

2. The combination of a shaded pole induction motor having a main winding adapted to be energized by alternating current and a pair of shading coils respectively adapted to cause shifting of the magnetic field around the rotor in the opposite direction, the resistance of one of the shading coil circuits being substantially greater than the other whereby to cause rotation 01 the motor armature in one direction when the circuit 01. higher resistance is closed and in the other direction under the predominating shading eii'ect oi the other coil when both of the coils ar effective.

3. The combination of a shaded pole induction motor having a main winding adapted to be energized by alternating current, a pair of shading coils each comprising a plurality of turns and respectively adapted when short-circuited to cause shifting of the magnetic field around the rotor in opposite directions, and selectively operable means by which the resistance of the circuit through either of said coils may be increased substantially relative to that of the circuit through the other coil whereby said motor will operate in one direction under the predominating shading effect of one of said coils when both of said circuits are closed.

4. The combination of a shaded pole induction motor having a main winding adapted to be energized by alternating current, a pair oi shading coils each comprising a plurality of turns and respectively adapted when short-circuited to cause shifting of the magnetic field around the rotor in opposite directions, the circuit through one of said shading coils being of substantially higher resistance than the circuit through the other coil, and means by which said high resistance circuit or both oi said circuits may be closed selectively.

5. A reversible single phase alternating current motor having poles, one of which is divided into two pole sectionaa shading means on one of said pole sections of sufllcient shading capacity to produce phase splitting causing rotation of the motor in one direction, and 9. normally open circuited shading means on the other section of the same pole having sufliciently larger shading capacity to act in conjunction with the first named shaded pole section but in opposition to the first named shading means to produce reverse phasesplitting and cause the motor to rotate in a reverse direction when the circuit of the last named shading means is closed.

6. ihe combination oi a reversible shaded pole induction motor having a stator, a rotor, a primary winding on the stator, and two shading means respectively tending when energized to eil'ect rotation of the rotor in opposite directions, and a control circuit having a movable controlling element, one of said shading means being effective upon a movement of said element -to exert a sumciently greater shading effect than the other shading means to cause rotation of the rotor in one direction with both of said shading means energized.

7. The combination of a reversible shaded pole motor having a rotor, a stator having a primary winding, and two shading means on said stator adapted when energized to cause phase splitting tending to produce rotation of the rotor in opposite directions, and means controlling at least one of said shading means so as to efl'ect rotation of the rotor in one direction with both of said shading means energized and acting in opposition to'cachother.

8. vThe combination of a reversible shaded pole motor comprising a rotor, a stator having a primary winding, and two shading means adapted to cause rotation of the rotor in opposite directions, at least one of said shading means having a controllable external circuit by which the relative shading effects of, the two shading means may be varied, and means controlling said circuit to cause one 0! said shading means to exert a predominating efl'ect over the other to cause rotation of the rotor in one direction when both shading means are energized.

9. A reversible shaded pole motor comprising a rotor, a stator having a primary winding, and two shading means on the stator adapted when energized to cause phase splitting tending to produce rotation of the rotor in opposite directions, at least one of said shading means comprising a multiple turn coil, means externally of said coil controlling the circuit thereof to vary the relative shading effects of the two shading means for producing rotation of the rotor in either direction, both of said shading means. being adapted to be energized simultaneously with one of said means exerting a predominating effect over the otherto cause rotation oi. the rotor in one of said directions.

10. Areversible shaded pole motor comprising a rotor, a stator having a primary winding, two shading means on the stator adapted to cause phase splitting tending to produce rotation of the rotor in opposite directions, and selectively operable means controlling at least one of said shading means to cause rotation of the rotor in one direction with a predetermined torque or to cause both of said shading means to be energized simultaneously with one of the shading means exerting a predominating efl'ect over the other so as to induce rotation of the rotor in the opposite direction with a torque approxio mately equal to said first mentioned torque.

GEORGE ronnEs'r mum 

